Articles known as pneumatic or gas springs, which for convenience can be referred to as counterbalance links have become common in commercial articles, particularly in the automotive industry, but they are being used in many other fields wherever the need is present to provide a counterbalance force for closure units, such as lids, doors and cabinet fronts, and gas spring replacement for mechanical spring fittings has become prevalent. In some fields of use particularly in the automotive field where the pneumatic springs are used on each side of trunk lids and hatch backs the springs counterbalance the weight of the lid or hatch back. They are compressed when the lids are closed and extend under differential pressure force acting on the piston when the lid is opened. The control orifice bypass effectively slows the opening speed of the lid. As improvements have occurred, it became desirable to utilize the small amount of lubricating oil, included in the springs, at the rod side of the piston to create a dampening action as the spring approaches the end of the extension stroke.
In automotive installations, particularly on hatchbacks, the counterbalance tilts back and forth between a shaft up angle and a shaft down angle. When the counterbalance is in the shaft up position any of the lubricating oil trapped above the piston slowly flows down below the piston, thus there is no oil on the shaft side of the piston to provide damping at the end of the expansion stroke.
Examples of prior art pneumatic counterbalance which involve damping of the shaft stroke are seen in the following U.S. Patents: U.S. Pat. No. 4,221,367 to L. O. Smith for Gas Spring With Two stage Damping; and U.S. Pat. No. 4,310,148 to H. Freitag for Cylinder and Piston Device.